Usually the surfaces of a combustion chamber exposed to the hot gas are cooled by means of impingement cooling, wherein the cooling medium used for cooling impinges practically vertically on a surface to be cooled. This type of cooling is very effective, but the cooling medium experiences a large pressure loss in the process as a result of the impingement on the surface to be cooled.
In a gas turbine, air taken from an air stream generated by a compressor is normally used as the cooling medium. If impingement cooling is the main cooling method employed here, then, owing to the large pressure loss occurring in the process, once cooling is complete, the air used in the process normally can no longer be used for the combustion because the mass flow rate of the cooling air is too low after the cooling. Thus once cooling is complete, the cooling air is no longer available for the combustion. This means there is no alternative but to accept a loss of air that typically ranges from 4 to 8% of the mass flow rate generated by a compressor. In addition, such a loss of air leads to a reduction in the efficiency of the turbine.
DE 197 51 299 C2 discloses a combustion chamber that has a wall structure surrounding an internal area and an inner wall offset from this wall structure.
An intermediate wall having orifices through which cooling steam passes for impingement cooling of the inner wall is also arranged in the intermediate area formed by the wall structure and the inner wall.
The combustion chamber is steam cooled, wherein the cooling steam enters an outer cooling area, passes from here through the orifices into an inner cooling area and there cools by impingement cooling the side of the inner wall not facing the hot gas.
The disadvantage here is that the cooling medium—cooling steam in this case—suffers a large loss in pressure as a result of the impingement cooling. If cooling air were to be used instead of cooling steam in the combustion chamber described, then the cooling air flow would no longer be usable for the combustion because of the large pressure loss.